Modernize views, view controllers, and other classes.

* Use properties where possible, and avoid declaring ivars
* Replace most nibs by using Storyboards
* Replace FlexiCell and supporting classes with one class that
uses Auto Layout
* Make cells for displaying tasks using Auto Layout and ReactiveCocoa
* Reorganize project structure to group classes by role
* Update README.md with instructions for init'ing the ReactiveCocoa
submodule and adding a DropboxApiKey.h file
* Add ReactiveCocoa v1.9.6, as a submodule, and use it to simplify use of
DropboxFileDownloader & Uploader

Also add a view for filtering tasks.